A non-contact method for efficient, non-invasive excitation of mechanical waves in soft media is proposed, in which we focus an ultrasound (US) signal through air onto the surface of a medium under study. The US wave reflected from the air/medium interface provides radiation force to the medium surface that launches a transient mechanical wave in the transverse (lateral) direction. The type of mechanical wave is determined by boundary conditions. To prove this concept, a home-made 1 MHz piezo-ceramic transducer with a matching layer to air sends a chirped US signal centered at 1 MHz to a 1.6 mm thick gelatin phantom mimicking soft biological tissue. A phase-sensitive (PhS)-optical coherence tomography system is used to track/image the mechanical wave. The reconstructed transient displacement of the mechanical wave in space and time demonstrates highly efficient generation, thus offering great promise for non-contact, non-invasive characterization of soft media, in general, and for elasticity measurements in delicate soft tissues and organs in bio-medicine, in particular.

Adhesively bonded metals are increasingly used in many industries. Inspecting these parts remains challenging for modern non-destructive testing techniques. Laser ultrasound (LU) has shown great potential in high-resolution imaging of carbon-reinforced composites. For metals, excitation of longitudinal waves is inefficient without surface ablation. However, shear waves can be efficiently generated in the thermo-elastic regime and used to image defects in metallic structures. Here we present a compact LU system consisting of a high repetition rate diode-pumped laser to excite shear waves and noncontact detection with a highly sensitive fiber optic Sagnac interferometer to inspect adhesively bonded aluminum plates. Multiphysics finite difference simulations are performed to optimize the measurement configuration. Damage detection is performed for a structure consisting of three aluminum plates bonded with an epoxy film. Defects are simulated by a thin Teflon film. It is shown that the proposed technique can efficiently localize defects in both adhesion layers.

Ultrasonic phased arrays have a considerable application potential for structural health monitoring (SHM) of planar structures. 2D topologies are required to obtain unequivocal damage localization in these applications. Since array performance depends, among other factors, on its topology, there is a demand for robust and numerically efficient tools for arrays design. To deal with this problem we propose a method based on frequency-dependent structure transfer function that can deal with multi-modal and dispersive nature of Lamb waves as well as with the arbitrary shape of the excitation bursts. The proposed method is validated using an accurate numerical model of waves propagation. Finally, an efficient experimental method is proposed employing virtual array implemented by means of scanning laser vibrometer for sensing the Lamb waves in multiple points corresponding to the locations of the 2D array elements. In this way the performance of various array topologies can be evaluated experimentally in the reception mode without the need for a physical prototype.